Bob Scott

I'd like to understand electromagnetics a bit better. What is the definition of electromagnetic flux? I'm attempting to make a better equation for flux density.

Is flux static, like a measure of the magnetic field intensity, as the lines of force that you can see using iron filings over a bar magnet?

Or is it defined as dynamic only, applying to AC fields only? The equation for B (AC flux density) in Gauss in my Dad's old engineering books applies to AC voltages. These books are really terse.

The ambiguity is in the word "flux" itself. When something is in a state of flux it is changing.

Papabravo

Flux is like fluid flow. It is a characteristic of vector fields.

A fluid flowing through a pipe is a vector field, that is there is a velocity vector at each point in the field.
An electric field is a vector field, that is there is an electric field vector at each point in the field.
A magnetic field is a vector field, that is there is a magnetic field vector at each point in the field.
Atmospheric temperature is a scalar field, that is there is a scalar quantity at each point in the field.

Vector fields exert forces on real particles moving through them, scalar fields do not.

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Sceadwian

Flux does not always mean change. There are of course static electric fields, lightning being an obvious example. There are static magnetic fields, any polarized magnetic material is an example. One of the meanings for flux under the physics subdefinition of flux according to dictionary.com is:

A quantity expressing the strength of a field of force in a given area.

Static electric flux is easy to define, the number of electrons in a given area, moveable flux is different as the bulk of electrons in matter are all 'in use' so the net electric flux is zero. Magnetic flux is a bit more complex, I know little to nothing about exactly what gives rise to a static magnetic field, something I should probably look up one day. Electromagnetic theory is based primarily on the interaction of electric and magnetic fields which implies them changing, and non-moving electric or magnetic flux is pretty boring stuff.

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Bob Scott

Thank you very much Papabravo and Sceadwian! That clears that up for me.

Papabravo, thanks for refreshing vector/scalars for me. Thanks Sceadwian for seeing it at a different angle. The more points of view that you can get of a subject, the easier it is to understand intuitively.

microtexan

Why are you trying to make a better equation for flux density?

Bob Scott

I thought it would be easier to design a saturating switching power supply transformer with a different equation for B.
The existing B=V*10^8/(4.44*A*N*f) uses AC voltage and frequency. I think current is more suitable.

So I've come up with some new ones:

B=0.1415*Al*N*I/A

B is in Gauss
Al is the core's inductance in milliHenries at 1000 turns.
N is the number of turns that pass through the core.
A is the core's cross sectional area in cm^2.

I've also come up with:

Al=µ *0.89*A/Le

µ = the core material permeability.
A is the core's cross sectional area in cm^2.
Le is the core's magnetic path length in cm.

This way you can easily see that the Al value is dependent only on core material and core geometry.

You see I had some trouble verifying the published specifications of an Amidon toroidal core FT240-77 made of type 77 material that has a characteristic permeability of 2000. The specs in Amidon's leaflet said it had an Al value of 3130. Their catalog said it was about 2750. My own test with an oscillator to determine inductance found the real thing to be closer to 1690.

I can elaborate on the derivation of the formulas but only if you're interested.

Papabravo

I don't mean to be impertinent but a saturated inductor in an SMPS sounds like a really bad idea. Are you sure that is what you want to do?

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Bob Scott

Oh, you would never sound impertinent.

The saturation is part of the oscillation cycle. As soon as the core saturates, the driving transistor turns off. It's been done before in photographic Xenon flash units and... remember the old Mark 10 capacitive discharge ignition? See:

Attached Images

mark10.jpg (74.3 KB, 34 views)

BaCaRdi

Sorry to post as this is out of my league, but caught my eye, as I need more Theory work to understand everything I am learning. I am a hobbyist so take it easy on me..lol

The example given the Xenon flash, sounds allot like a Florecent starter. Right at saturation it disconnects the current there-by causing the feild to breakdown. Is the math involved based on Maxwell's equations?

Am I close on my assumptions?

The math escapes me, sorry to admit.


-BaC

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BaCaRdi

This sites is on my quick list, nicely layout as well. I am sure it has been posted here already so pardon me if it was;

Magnetic Flux

-BaC

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DigiTan

That's correct. The rule comes from Faraday's and Lenz's Laws. Faraday's Law says if a voltage on a wire loop varies in time: then so will the flux through that wire loop. And vice versa.

Lenz's Law states if a field expands or breaks down, a new current will appear and oppose the change in flux. This current moves in the opposite direction from the one that energized the coil in the first place. That's why you have to put reverse-protection diodes on relays.

Take a generator for example. Turning it produces a voltage to power your stuff. But at the same time, you are changing the flux and introducing the opposition current, which in turn opposes your attempts to change the flux (in a recursive relationship). If the generator is not connected to anything, the opposition current can't flow and the generator is easy to turn. But if you short the generator with a wire, the opposition current can flow strongly and the generator gets harder to rotate. Electric motors do it too. If you want to stop a motor fast, you short its terminals.

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BaCaRdi

Thank you DigiTan

-BaC

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indulis

It's called a "Mag Amp"

You should be able to find what your looking for here:

http://focus.ti.com/lit/ml/slup129/slup129.pdf

Bob Scott

Hey- I'm still confused. I extrapolated on the available equations for B and H, and L etc WHICH HAPPEN TO BE LINEAR EQUATIONS. But the B-H curve is far from linear. There is definitely a whooshing sound over my head.

Ditto

arrie

Sorry to barge in on your topic, but it's kinda related.
Here follows a quiz, one I'm not quite solving yet:
So you connect a 220V 125Hz supply across a coil, the current in the coil reaches 692mA after 3.2ms, and then the circuit is switched off, probably using a switch between the supply and coil - hehehe......
What would the final steady value of current be?
Any educated guesses?